U.S. patent application number 14/664007 was filed with the patent office on 2015-07-09 for inspection apparatus to detect a target located within a pattern for lithography.
This patent application is currently assigned to ASML Netherlands B.V.. The applicant listed for this patent is ASML Netherlands B.V.. Invention is credited to Marcus Adrianus VAN DE KERKHOF.
Application Number | 20150192858 14/664007 |
Document ID | / |
Family ID | 40823592 |
Filed Date | 2015-07-09 |
United States Patent
Application |
20150192858 |
Kind Code |
A1 |
VAN DE KERKHOF; Marcus
Adrianus |
July 9, 2015 |
Inspection Apparatus to Detect a Target Located Within a Pattern
for Lithography
Abstract
A system detects targets located within patterns. It operates in
the pupil plane by filtering the received signal from the
surrounding pattern. A method includes illuminating a target and a
surrounding pattern with radiation, detecting the radiation
reflected by the target and the surrounding pattern and forming a
first set of data based on the detected radiation, removing
portions of the first set of data which correspond to the target to
form reduced data, interpolating the remaining portions of the
reduced data over the removed portions to form product data, and
subtracting the product data from the first set of data to form
target data.
Inventors: |
VAN DE KERKHOF; Marcus
Adrianus; (Helmond, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASML Netherlands B.V. |
Veldhoven |
|
NL |
|
|
Assignee: |
ASML Netherlands B.V.
Veldhoven
NL
|
Family ID: |
40823592 |
Appl. No.: |
14/664007 |
Filed: |
March 20, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12989902 |
Jan 5, 2011 |
8988658 |
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PCT/EP2009/003051 |
Apr 27, 2009 |
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14664007 |
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61071673 |
May 12, 2008 |
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Current U.S.
Class: |
355/67 ; 356/369;
356/399 |
Current CPC
Class: |
G01N 2201/10 20130101;
G01N 21/9501 20130101; G01N 21/956 20130101; G03F 7/70616 20130101;
G01N 2201/06113 20130101; G01N 21/211 20130101; G03F 7/70633
20130101 |
International
Class: |
G03F 7/20 20060101
G03F007/20; G01N 21/956 20060101 G01N021/956; G01N 21/21 20060101
G01N021/21 |
Claims
1. A method of measuring a target on a substrate, wherein the
target is located within a surrounding pattern on the substrate,
the method comprising: illuminating the target and the surrounding
pattern with radiation; detecting the radiation reflected by the
target and the surrounding pattern and forming a first set of data
based on the detected radiation; removing portions of the first set
of data which correspond to the target to form reduced data;
interpolating the remaining portions of the reduced data over the
removed portions to form product data; and subtracting the product
data from the first set of data to form target data.
2. The method according to claim 1, wherein the detecting comprises
detecting the radiation in the pupil plane
3. The method according to claim 1, wherein the detecting comprises
performing a Fourier transform on reflected radiation data to form
the first set of data.
4. The method according to claim 1, further comprising analyzing
the target data to determine an overlay error, or a critical
dimension or a shape of a feature, or any combination of the
foregoing.
5. An inspection apparatus comprising: a radiation projector
configured to illuminate a target and a pattern surrounding the
target on a substrate with radiation; a detector configured to
detect the radiation reflected from the target and the pattern
surrounding the target, the detected radiation being used to form a
first set of data; and a processor configured to: remove portions
of the first set of data which correspond to the target to form
reduced data; interpolate the remaining portions of the reduced
data over the removed portions to form product data; and subtract
the product data from the first set of data to form target
data.
6. The inspection apparatus according to claim 5, wherein the
inspection apparatus comprises an angle resolved scatterometer.
7. The inspection apparatus according to claim 5, wherein the
inspection apparatus comprises an ellipsometer.
8. A lithographic apparatus comprising: a projection system
configured to project an image of a pattern on to a substrate; and
an inspection apparatus configured to measure a target on the
substrate, wherein the target is located within a surrounding
pattern on the substrate, the inspection apparatus comprising: a
radiation projector configured to illuminate the target and the
surrounding pattern with radiation; a detector configured to detect
the radiation reflected from the target and the surrounding
pattern, the detected radiation being used to form a first set of
data; and a processor configured to: remove portions of the first
set of data which correspond to the target to form reduced data;
interpolate the remaining portions of the reduced data over the
removed portions to form product data; and subtract the product
data from the first set of data to form target data.
9. A data processor configured to process a first set of data to
measure a target on a substrate, wherein the target is located
within a surrounding pattern on the substrate, the data processor
being configured to: remove portions of the first set of data which
correspond to the target to form reduced data; interpolate the
remaining portions of the reduced data over the removed portions to
form product data; and subtract the product data from the first set
of data to form target data.
10. A method of measuring a target on a substrate, the substrate
comprising a pattern and the target, the method comprising:
illuminating the pattern and the target with radiation; detecting
the radiation reflected by the pattern and the target; performing a
first Fourier transform on the detected radiation to form Fourier
transform data; performing a second Fourier transform on the known
pattern to form pattern Fourier transform data; and subtracting the
pattern Fourier transform data from the Fourier transform data to
form target data.
11. The method according to claim 10, further comprising analyzing
the target data to determine an overlay error, or a critical
dimension or a shape of a feature, or any combination of the
foregoing.
12. An inspection apparatus configured to measure a target on a
substrate, the substrate comprising a pattern and the target, the
apparatus comprising: a radiation projector configured to
illuminate the pattern and the target with radiation; a detector
configured to detect the radiation reflected from the known pattern
and the target; and a processor configured to perform a first
Fourier transform on the detected radiation to form Fourier
transform data, perform a second Fourier transform on the known
pattern to form pattern Fourier transform data, and subtract the
pattern Fourier transform data from the Fourier transform data to
form target data.
13. The inspection apparatus according to claim 12, wherein the
inspection apparatus comprises an angle resolved scatterometer.
14. The inspection apparatus according to claim 12, wherein the
inspection apparatus comprises an ellipsometer.
15. A lithographic apparatus comprising: a projection system
configured to project an image of a pattern on to a substrate; and
an inspection apparatus configured to measure a target on the
substrate, the substrate comprising a pattern and the target, the
inspection apparatus comprising: a radiation projector configured
to illuminate the pattern and the target with radiation; a detector
configured to detect the radiation reflected from the pattern and
the target; and a data processor configured to: perform a first
Fourier transform on the detected radiation to form Fourier
transform data, perform a second Fourier transform on the known
pattern to form pattern Fourier transform data, and subtract the
pattern Fourier transform data from the Fourier transform data to
form target data.
16. A processor configured to process Fourier transform data to
measure a target on a substrate, the substrate comprising a pattern
and the target, the processor being configured to: perform a first
Fourier transform on detected radiation from the pattern and the
target to form Fourier transform data; perform a second Fourier
transform on the pattern to form pattern Fourier transform data;
and subtract the pattern Fourier transform data from the Fourier
transform data to form target data.
17. A method for determining a symmetry of conformal coatings on a
substrate comprising a sacrificial feature, the method comprising:
applying a conformal coating to a substrate comprising the
sacrificial feature; etching the conformal coating to reveal the
feature; removing the feature to leave a conformal feature;
illuminating the substrate with radiation; detecting the radiation
reflected by the substrate to form reflected radiation data;
removing portions of the reflected radiation data which correspond
to the conformal feature to form reduced data; interpolating the
remaining portions of the reduced data over the removed portions to
form product data; and subtracting the product data from the
reflected radiation data to form target data.
18. The method according to claim 17, further comprising forming
the sacrificial feature on the substrate.
19. A method for determining a symmetry of conformal coatings on a
substrate comprising a pattern and a sacrificial feature, the
method comprising: applying a conformal coating to the substrate
comprising the sacrificial feature; etching the conformal coating
to reveal the feature; removing the feature to leave a conformal
feature; illuminating the substrate with radiation; detecting the
radiation reflected by the substrate; performing a first Fourier
transform on the detected radiation to form Fourier transform data;
performing a second Fourier transform on the pattern to form
pattern Fourier transform data; and subtracting the pattern Fourier
transform data from the Fourier transform data to form target
data.
20. The method according to claim 19, further comprising forming
the sacrificial feature on the substrate.
Description
[0001] This application is related to U.S. application Ser. No.
12/989,902, filed Jan. 5, 2011, PCT App. PCT/EP2009/003051 filed on
Apr. 27, 2009, and U.S. Prov. Appl. No. 61/071,673, filed on May
12, 2008, which are all incorporated by reference herein in their
entireties.
FIELD
[0002] The present invention relates to methods of inspection
usable, for example, in the manufacture of devices by lithographic
techniques and to methods of manufacturing devices using
lithographic techniques.
BACKGROUND
[0003] A lithographic apparatus is a machine that applies a desired
pattern onto a substrate, usually onto a target portion of the
substrate. A lithographic apparatus can be used, for example, in
the manufacture of integrated circuits (ICs). In that instance, a
patterning device, which is alternatively referred to as a mask or
a reticle, may be used to generate a circuit pattern to be formed
on an individual layer of the IC. This pattern can be transferred
onto a target portion (e.g. including part of, one, or several
dies) on a substrate (e.g. a silicon wafer). Transfer of the
pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In
general, a single substrate will contain a network of adjacent
target portions that are successively patterned. Known lithographic
apparatus include so-called steppers, in which each target portion
is irradiated by exposing an entire pattern onto the target portion
at one time, and so-called scanners, in which each target portion
is irradiated by scanning the pattern through a radiation beam in a
given direction (the "scanning"-direction) while synchronously
scanning the substrate parallel or anti-parallel to this direction.
It is also possible to transfer the pattern from the patterning
device to the substrate by imprinting the pattern onto the
substrate.
[0004] In order to monitor the lithographic process, it is
desirable to measure parameters of the patterned substrate, for
example the overlay error between successive layers formed in or on
it. There are various techniques for making measurements of the
microscopic structures formed in lithographic processes, including
the use of scanning electron microscopes and various specialized
tools. One form of specialized inspection tool is a scatterometer
in which a beam of radiation is directed onto a target on the
surface of the substrate and properties of the scattered or
reflected beam are measured. By comparing the properties of the
beam before and after it has been reflected or scattered by the
substrate, the properties of the substrate can be determined. This
can be done, for example, by comparing the reflected beam with data
stored in a library of known measurements associated with known
substrate properties. Two main types of scatterometer are known.
Spectroscopic scatterometers direct a broadband radiation beam onto
the substrate and measure the spectrum (intensity as a function of
wavelength) of the radiation scattered into a particular narrow
angular range. Angularly resolved scatterometers use a
monochromatic radiation beam and measure the intensity of the
scattered radiation as a function of angle.
[0005] In scatterometers and lithographic apparatus, targets are
used in the determination of overlay errors. These are
conventionally positioned in the scribe lanes between the patterns.
The overlay error at the target site is thus measured. However, the
overlay error at the position of the pattern is therefore an
interpolation between the overlay at different points surrounding
the pattern.
[0006] Although the targets could be positioned within the patterns
themselves, this is not desirable because the targets used are
relatively large and therefore take up too much of the area that is
designed for product patterns, thereby compromising device
functionality.
SUMMARY
[0007] It is desirable to provide a method of measuring a target
which is sufficiently small to be placed on the substrate within
the pattern.
[0008] According to an aspect of the invention, there is provided
an inspection apparatus, lithographic apparatus or lithographic
cell configured to measure a property of a substrate.
[0009] According to an aspect of the invention, there is provided a
method of measuring a target on a substrate, the method including
projecting radiation onto a substrate; detecting the radiation
reflected by the substrate and forming a set of fourier transform
data based on the detected radiation; removing portions of the
fourier transform data which correspond to the target to form
reduced fourier transform data; interpolating the portions of the
reduced fourier transform data which were removed, to form product
fourier transform data; and subtracting the product fourier
transform data from the fourier transform data to form target
data.
[0010] According to an embodiment of the invention, there is
provided an inspection apparatus configured to measure a target on
a substrate, the apparatus including a radiation projector
configured to illuminate the substrate with radiation; a high
numerical aperture lens; a detector configured to detect the
radiation reflected from a surface of the substrate, the detected
radiation being used to form fourier transform data; and a data
processor configured to remove portions of the fourier transform
data which correspond to the target to form reduced fourier
transform data; interpolate the portions of the removed reduced
fourier transform data to form product fourier transform data; and
subtract the product fourier transform data from the fourier
transform data to form target data.
[0011] According to an aspect of the invention, there is provided a
lithographic apparatus including a projection system configured to
project an image of a pattern on to a substrate; and an inspection
apparatus configured to measure a target on the substrate, the
inspection apparatus including a radiation projector configured to
illuminate the substrate with radiation; a high numerical aperture
lens; a detector configured to detect the radiation reflected from
a surface of the substrate, the detected radiation being used to
form fourier transform data; and a data processor configured to
remove portions of the fourier transform data which correspond to
the target to form reduced fourier transform data; interpolate the
portions of the removed reduced fourier transform data to form
product fourier transform data; and subtract the product fourier
transform data from the fourier transform data to form target
data.
[0012] According to an aspect of the invention, there is provided a
method of measuring a target on a substrate, the substrate
including a known pattern and the target, the method including
illuminating the substrate with radiation; detecting the radiation
reflected by the substrate to form a fourier transform data;
performing a fourier transform on the known pattern to form pattern
fourier transform data; and subtracting the pattern fourier
transform data from the fourier transform data to form target
data.
[0013] According to an aspect of the invention, there is provided
an inspection apparatus configured to measure a target on a
substrate, the substrate including a known pattern and the target,
the apparatus including a radiation projector configured to
illuminate the substrate with radiation; a high numerical aperture
lens; a detector configured to detect the radiation reflected from
a surface of the substrate, the detected radiation being used to
form fourier transform data; and a data processor configured to
perform a fourier transform on the known pattern to form pattern
fourier transform data; and subtract the product fourier transform
data from the fourier transform data to form target data.
[0014] According to an aspect of the invention, there is provided
an inspection apparatus configured to measure a target on a
substrate, the substrate comprising a known pattern and the target,
the apparatus including a radiation projector configured to
illuminate the substrate with radiation; a high numerical aperture
lens; a detector configured to detect the radiation reflected from
a surface of the substrate, the detected radiation being used to
form fourier transform data; and a data processor configured to
perform a fourier transform on the known pattern to form pattern
fourier transform data; and subtract the product fourier transform
data from the fourier transform data to form target data.
[0015] According to an aspect of the invention, there is provided a
lithographic apparatus including a projection system configured to
project an image of a pattern on to a substrate; and an inspection
apparatus configured to measure a target on the substrate, the
substrate comprising a known pattern and the target, the inspection
apparatus including a radiation projector configured to illuminate
the substrate with radiation; a high numerical aperture lens; a
detector configured to detect the radiation reflected from a
surface of the substrate, the detected radiation being used to form
fourier transform data; and a data processor, the data processor
configured to perform a fourier transform on the known pattern to
form pattern fourier transform data; and subtract the pattern
fourier transform data from the fourier transform data to form
target data.
[0016] According to an aspect of the invention, there is provided a
method for determining a symmetry of conformal coatings on a
substrate comprising a sacrificial feature, the method including
applying a conformal coating to a substrate comprising the
sacrificial feature; etching the conformal coating to reveal the
feature; removing the feature to leave a conformal feature;
illuminating the substrate with radiation; detecting the radiation
reflected by the substrate to form reflected radiation data;
performing a fourier transform on the reflected radiation data to
form fourier transform data; removing portions of the fourier
transform data which correspond to the conformal feature to form
reduced fourier transform data; interpolating the portions of the
removed reduced fourier transform data which to form product
fourier transform data; and subtracting the product fourier
transform data from the fourier transform data to form target
data.
[0017] According to an aspect of the invention, there is provided a
method for determining a symmetry of conformal coatings on a
substrate including a known pattern and a sacrificial feature, the
method including applying a conformal coating to the substrate
comprising the sacrificial feature; etching the conformal coating
to reveal the feature; removing the feature to leave a conformal
feature; illuminating the substrate with radiation; detecting the
radiation reflected by the substrate, the reflected radiation being
used to form a set of fourier transform data; performing a fourier
transform on the known pattern to form pattern fourier transform
data; and subtracting the pattern fourier transform data from the
fourier transform data to form target data.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying schematic
drawings in which corresponding reference symbols indicate
corresponding parts, and in which:
[0019] FIG. 1 depicts a lithographic apparatus in accordance with
an embodiment of the invention;
[0020] FIG. 2 depicts a lithographic cell or cluster in accordance
with an embodiment of the invention;
[0021] FIG. 3 depicts a scatterometer in accordance with an
embodiment of the invention;
[0022] FIG. 4 depicts a scatterometer in accordance with an
embodiment of the invention;
[0023] FIG. 5 is a flow diagram in accordance with an embodiment of
the invention;
[0024] FIG. 6 is a flow diagram in accordance with an embodiment of
the invention; and
[0025] FIGS. 7a-d depict some of the procedures involved in an
embodiment of the invention.
DETAILED DESCRIPTION
[0026] FIG. 1 schematically depicts a lithographic apparatus. The
apparatus includes an illumination system (illuminator) IL
configured to condition a radiation beam B (e.g. UV radiation or
DUV radiation); a patterning device support or support structure
(e.g. a mask table) MT constructed to support a patterning device
(e.g. a mask) MA and connected to a first positioner PM configured
to accurately position the patterning device in accordance with
certain parameters; a substrate table or support (e.g. a wafer
table) WT constructed to hold a substrate (e.g. a resist-coated
wafer) W and connected to a second positioner PW configured to
accurately position the substrate in accordance with certain
parameters; and a projection system (e.g. a refractive projection
lens system) PL configured to project a pattern imparted to the
radiation beam B by patterning device MA onto a target portion C
(e.g. including one or more dies) of the substrate W.
[0027] The illumination system may include various types of optical
components, such as refractive, reflective, magnetic,
electromagnetic, electrostatic or other types of optical
components, or any combination thereof, to direct, shape, or
control radiation.
[0028] The patterning device support holds the patterning device in
a manner that depends on the orientation of the patterning device,
the design of the lithographic apparatus, and other conditions,
such as for example whether or not the patterning device is held in
a vacuum environment. The patterning device support can use
mechanical, vacuum, electrostatic or other clamping techniques to
hold the patterning device. The patterning device support may be a
frame or a table, for example, which may be fixed or movable as
required. The patterning device support may ensure that the
patterning device is at a desired position, for example with
respect to the projection system. Any use of the terms "reticle" or
"mask" herein may be considered synonymous with the more general
term "patterning device."
[0029] The term "patterning device" used herein should be broadly
interpreted as referring to any device that can be used to impart a
radiation beam with a pattern in its cross-section such as to
create a pattern in a target portion of the substrate. It should be
noted that the pattern imparted to the radiation beam may not
exactly correspond to the desired pattern in the target portion of
the substrate, for example if the pattern includes phase-shifting
features or so called assist features. Generally, the pattern
imparted to the radiation beam will correspond to a particular
functional layer in a device being created in the target portion,
such as an integrated circuit.
[0030] The patterning device may be transmissive or reflective.
Examples of patterning devices include masks, programmable mirror
arrays, and programmable LCD panels. Masks are well known in
lithography, and include mask types such as binary, alternating
phase-shift, and attenuated phase-shift, as well as various hybrid
mask types. An example of a programmable mirror array employs a
matrix arrangement of small mirrors, each of which can be
individually tilted so as to reflect an incoming radiation beam in
different directions. The tilted mirrors impart a pattern in a
radiation beam, which is reflected by the mirror matrix.
[0031] The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system,
including refractive, reflective, catadioptric, magnetic,
electromagnetic and electrostatic optical systems, or any
combination thereof, as appropriate for the exposure radiation
being used, or for other factors such as the use of an immersion
liquid or the use of a vacuum. Any use of the term "projection
lens" herein may be considered as synonymous with the more general
term "projection system".
[0032] As here depicted, the apparatus is of a transmissive type
(e.g. employing a transmissive mask). Alternatively, the apparatus
may be of a reflective type (e.g. employing a programmable mirror
array of a type as referred to above, or employing a reflective
mask).
[0033] The lithographic apparatus may be of a type having two (dual
stage) or more substrate tables (and/or two or more mask tables).
In such "multiple stage" machines the additional tables may be used
in parallel, or preparatory steps may be carried out on one or more
tables while one or more other tables are being used for
exposure.
[0034] The lithographic apparatus may also be of a type wherein at
least a portion of the substrate may be covered by a liquid having
a relatively high refractive index, e.g. water, so as to fill a
space between the projection system and the substrate. An immersion
liquid may also be applied to other spaces in the lithographic
apparatus, for example, between the mask and the projection system.
Immersion techniques are well known in the art for increasing the
numerical aperture of projection systems. The term "immersion" as
used herein does not mean that a structure, such as a substrate,
must be submerged in liquid, but rather only means that liquid is
located between the projection system and the substrate during
exposure.
[0035] Referring to FIG. 1, the illuminator IL receives a radiation
beam from a radiation source SO. The source and the lithographic
apparatus may be separate entities, for example when the source is
an excimer laser. In such cases, the source is not considered to
form part of the lithographic apparatus and the radiation beam is
passed from the source SO to the illuminator IL with the aid of a
beam delivery system BD including, for example, suitable directing
mirrors and/or a beam expander. In other cases the source may be an
integral part of the lithographic apparatus, for example when the
source is a mercury lamp. The source SO and the illuminator IL,
together with the beam delivery system BD if required, may be
referred to as a radiation system.
[0036] The illuminator IL may include an adjuster AD to adjust the
angular intensity distribution of the radiation beam. Generally, at
least the outer and/or inner radial extent (commonly referred to as
.sigma.-outer and .sigma.-inner, respectively) of the intensity
distribution in a pupil plane of the illuminator can be adjusted.
In addition, the illuminator IL may include various other
components, such as an integrator IN and a condenser CO. The
illuminator may be used to condition the radiation beam, to have a
desired uniformity and intensity distribution in its
cross-section.
[0037] The radiation beam B is incident on the patterning device
(e.g., mask) MA, which is held on the patterning device support
(e.g., mask table) MT, and is patterned by the patterning device.
Having traversed the patterning device (e.g. mask) MA, the
radiation beam B passes through the projection system PL, which
focuses the beam onto a target portion C of the substrate W. With
the aid of the second positioner PW and position sensor IF (e.g. an
interferometric device, linear encoder, 2-D encoder or capacitive
sensor), the substrate table WT can be moved accurately, e.g. so as
to position different target portions C in the path of the
radiation beam B. Similarly, the first positioner PM and another
position sensor (which is not explicitly depicted in FIG. 1) can be
used to accurately position the patterning device (e.g. mask) MA
with respect to the path of the radiation beam B, e.g. after
mechanical retrieval from a mask library, or during a scan. In
general, movement of the patterning device support (e.g. mask
table) MT may be realized with the aid of a long-stroke module
(coarse positioning) and a short-stroke module (fine positioning),
which form part of the first positioner PM. Similarly, movement of
the substrate table WT may be realized using a long-stroke module
and a short-stroke module, which form part of the second positioner
PW. In the case of a stepper (as opposed to a scanner) the
patterning device support (e.g. mask table) MT may be connected to
a short-stroke actuator only, or may be fixed. Patterning device
(e.g. mask) MA and substrate W may be aligned using patterning
device alignment marks M1, M2 and substrate alignment marks P1, P2.
Although the substrate alignment marks as illustrated occupy
dedicated target portions, they may be located in spaces between
target portions (these are known as scribe-lane alignment marks).
Similarly, in situations in which more than one die is provided on
the patterning device (e.g. mask) MA, the patterning device
alignment marks may be located between the dies.
[0038] The depicted apparatus could be used in at least one of the
following modes:
[0039] 1. In step mode, the patterning device support (e.g. mask
table) MT and the substrate table WT are kept essentially
stationary, while an entire pattern imparted to the radiation beam
is projected onto a target portion C at one time (i.e. a single
static exposure). The substrate table WT is then shifted in the X
and/or Y direction so that a different target portion C can be
exposed. In step mode, the maximum size of the exposure field
limits the size of the target portion C imaged in a single static
exposure.
[0040] 2. In scan mode, the patterning device support (e.g. mask
table) MT and the substrate table WT are scanned synchronously
while a pattern imparted to the radiation beam is projected onto a
target portion C (i.e. a single dynamic exposure). The velocity and
direction of the substrate table WT relative to the patterning
device support (e.g. mask table) MT may be determined by the
(de-)magnification and image reversal characteristics of the
projection system PL. In scan mode, the maximum size of the
exposure field limits the width (in the non-scanning direction) of
the target portion in a single dynamic exposure, whereas the length
of the scanning motion determines the height (in the scanning
direction) of the target portion.
[0041] 3. In another mode, the patterning device support (e.g. mask
table) MT is kept essentially stationary holding a programmable
patterning device, and the substrate table WT is moved or scanned
while a pattern imparted to the radiation beam is projected onto a
target portion C. In this mode, generally a pulsed radiation source
is employed and the programmable patterning device is updated as
required after each movement of the substrate table WT or in
between successive radiation pulses during a scan. This mode of
operation can be readily applied to maskless lithography that
utilizes programmable patterning device, such as a programmable
mirror array of a type as referred to above.
[0042] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0043] As shown in FIG. 2, the lithographic apparatus LA forms part
of a lithographic cell LC, also sometimes referred to a lithocell
or cluster, which also includes apparatus to perform pre- and
post-exposure processes on a substrate. Conventionally these
include spin coaters SC to deposit resist layers, developers DE to
develop exposed resist, chill plates CH and bake plates BK. A
substrate handler, or robot, RO picks up substrates from
input/output ports I/O1, I/O2, moves them between the different
process apparatus and delivers then to the loading bay LB of the
lithographic apparatus. These devices, which are often collectively
referred to as the track, are under the control of a track control
unit TCU which is itself controlled by the supervisory control
system SCS, which also controls the lithographic apparatus via
lithography control unit LACU. Thus, the different apparatus can be
operated to maximize throughput and processing efficiency.
[0044] In order that the substrates that are exposed by the
lithographic apparatus are exposed correctly and consistently, it
is desirable to inspect exposed substrates to measure properties
such as overlay errors between subsequent layers, line thicknesses,
critical dimensions (CD), etc. If errors are detected, adjustments
may be made to exposures of subsequent substrates, especially if
the inspection can be done soon and fast enough that other
substrates of the same batch are still to be exposed. Also, already
exposed substrates may be stripped and reworked--to improve
yield--or discarded--thereby avoiding performing exposures on
substrates that are known to be faulty. In a case where only some
target portions of a substrate are faulty, further exposures can be
performed only on those target portions which are good.
[0045] An inspection apparatus is used to determine the properties
of the substrates, and in particular, how the properties of
different substrates or different layers of the same substrate vary
from layer to layer. The inspection apparatus may be integrated
into the lithographic apparatus LA or the lithocell LC or may be a
stand-alone device. To enable most rapid measurements, it is
desirable that the inspection apparatus measure properties in the
exposed resist layer immediately after the exposure. However, the
latent image in the resist has a very low contrast--there is only a
very small difference in refractive index between the parts of the
resist which have been exposed to radiation and those which have
not--and not all inspection apparatus have sufficient sensitivity
to make useful measurements of the latent image. Therefore
measurements may be taken after the post-exposure bake step (PEB)
which is customarily the first step carried out on exposed
substrates and increases the contrast between exposed and unexposed
parts of the resist. At this stage, the image in the resist may be
referred to as semi-latent. It is also possible to make
measurements of the developed resist image--at which point either
the exposed or unexposed parts of the resist have been removed--or
after a pattern transfer step such as etching. The latter
possibility limits the possibilities for rework of faulty
substrates but may still provide useful information.
[0046] FIG. 3 depicts a scatterometer SM1 which may be used in an
embodiment of the present invention. It includes a broadband (white
light) radiation projector 2 which projects radiation onto a
substrate W. The reflected radiation is passed to a spectrometer
detector 4, which measures a spectrum 10 (intensity as a function
of wavelength) of the specular reflected radiation. From this data,
the structure or profile giving rise to the detected spectrum may
be reconstructed by processing unit PU, e.g. by Rigorous Coupled
Wave Analysis and non-linear regression or by comparison with a
library of simulated spectra as shown at the bottom of FIG. 3. In
general, for the reconstruction the general form of the structure
is known and some parameters are assumed from knowledge of the
process by which the structure was made, leaving only a few
parameters of the structure to be determined from the scatterometry
data. Such a scatterometer may be configured as a normal-incidence
scatterometer or an oblique-incidence scatterometer.
[0047] Another scatterometer SM2 that may be used with an
embodiment of the present invention is shown in FIG. 4. In this
device, the radiation emitted by radiation source 2 is focused
using lens system 12 through interference filter 13 and polarizer
17, reflected by partially reflected surface 16 and is focused onto
substrate W via a microscope objective lens 15, which has a high
numerical aperture (NA), preferably at least 0.9 and more
preferably at least 0.95. Immersion scatterometers may even have
lenses with numerical apertures over 1. The reflected radiation
then transmits through partially reflective surface 16 into a
detector 18 in order to have the scatter spectrum detected. The
detector may be located in the back-projected pupil plane 11, which
is at the focal length of the lens system 15, however the pupil
plane may instead be re-imaged with auxiliary optics (not shown)
onto the detector. The pupil plane is the plane in which the radial
position of radiation defines the angle of incidence and the
angular position defines azimuth angle of the radiation. The
detector is preferably a two-dimensional detector so that a
two-dimensional angular scatter spectrum of a substrate target 30
can be measured. The detector 18 may be, for example, an array of
CCD or CMOS sensors, and may use an integration time of, for
example, 40 milliseconds per frame.
[0048] A reference beam is often used for example to measure the
intensity of the incident radiation. To do this, when the radiation
beam is incident on the beam splitter 16 part of it is transmitted
through the beam splitter as a reference beam towards a reference
mirror 14. The reference beam is then projected onto a different
part of the same detector 18.
[0049] A set of interference filters 13 is available to select a
wavelength of interest in the range of, say, 405-790 nm or even
lower, such as 200-300 nm. The interference filter may be tunable
rather than including a set of different filters. A grating could
be used instead of interference filters.
[0050] The detector 18 may measure the intensity of scattered light
at a single wavelength (or narrow wavelength range), the intensity
separately at multiple wavelengths or integrated over a wavelength
range. Furthermore, the detector may separately measure the
intensity of transverse magnetic- and transverse electric-polarized
light and/or the phase difference between the transverse magnetic-
and transverse electric-polarized light.
[0051] Using a broadband light source (i.e. one with a wide range
of light frequencies or wavelengths--and therefore of colors) is
possible, which gives a large etendue, allowing the mixing of
multiple wavelengths. The plurality of wavelengths in the broadband
preferably each has a bandwidth of .delta..lamda. and a spacing of
at least 2 .delta..lamda. (i.e. twice the bandwidth). Several
"sources" of radiation can be different portions of an extended
radiation source which have been split using fiber bundles. In this
way, angle resolved scatter spectra can be measured at multiple
wavelengths in parallel. A 3-D spectrum (wavelength and two
different angles) can be measured, which contains more information
than a 2-D spectrum. This allows more information to be measured
which increases metrology process robustness. This is described in
more detail in EP1,628,164A.
[0052] The target 30 on substrate W may be a grating, which is
printed such that after development, the bars are formed of solid
resist lines. The bars may alternatively be etched into the
substrate. This pattern is sensitive to chromatic aberrations in
the lithographic projection apparatus, particularly the projection
system PL, and illumination symmetry and the presence of such
aberrations will manifest themselves in a variation in the printed
grating. Accordingly, the scatterometry data of the printed
gratings is used to reconstruct the gratings. The parameters of the
grating, such as line widths and shapes, may be input to the
reconstruction process, performed by processing unit PU, from
knowledge of the printing step and/or other scatterometry
processes.
[0053] An embodiment of the invention allows smaller targets to be
more accurately measured. Thus targets used in conjunction with an
embodiment of the invention may be approximately 10 .mu.m.times.10
.mu.m. When the radiation is focused on the target, there will
additionally be diffraction from the surrounding pattern. In an
embodiment of the invention, the diffraction from the surrounding
pattern (in the pupil plane) is filtered out such that only the
portions from the target remain.
[0054] An embodiment of the invention operates in the pupil plane
and includes the following procedures, as shown in FIG. 5:
[0055] a) obtaining fourier transform data, S1;
[0056] b) removing portions of the Fourier transform corresponding
to the target, S2;
[0057] c) interpolating the remaining fourier transform over the
removed portions, S3; and
[0058] d) subtracting the fourier transform of procedure (c) from
the fourier transform of procedure (a), S4.
[0059] Procedure (a) may be achieved by placing the detector in the
pupil plane (or alternatively by detecting data and performing a
fourier transform). Then, based on the aperture, pitch and
orientation of the target the portions of the fourier transform
data corresponding to the target can be removed. The procedure of
obtaining transform data and removing portions of the Fourier
transform may be carried out by a calculator provided in the
scatterometer SM1 or SM2. The targets generally have a pitch of
about 500-1000 nm, whereas the surrounding pattern has a much
smaller pitch. Thus, if radiation of a suitable wavelength is used
and combined with a suitable numerical aperture there will first
order contributions only from the target. There may additionally be
some lower intensity scattering from the surrounding patterns,
which procedure (c) is intended to estimate. Any knowledge of the
surrounding pattern may be used to improve the interpolation of
procedure (c). Procedure (c) estimates the cross talk from the
surrounding pattern. By subtracting the fourier transform of the
surrounding pattern (including estimated cross talk) from the
original fourier transform data the fourier transform of the target
remains. The target data remaining may then be used to calculate
the overlay error, or for any other purposes. The procedures a-d,
or part thereof, may be carried out with the use of a data
processor.
[0060] In another embodiment of the invention the fourier transform
of the pattern (excluding the target) on the substrate is known. An
embodiment of the invention includes the following procedures, as
shown in FIG. 6:
[0061] (a) obtaining fourier transform data, S11;
[0062] (b) performing a fourier transform on the known pattern,
S12; and
[0063] (c) subtracting the fourier transform of procedure (b) from
the fourier transform of procedure (a), S14.
[0064] This method avoids the need for approximating the cross talk
from the pattern and the lower intensity scattering by using the
fourier transform of the known pattern. Thus, in this embodiment a
larger angular spread may be used and a target with a larger
overlay range may be detected. However, this method relies on the
structure of the surrounding pattern being known.
[0065] According to a further embodiment of the invention, the
method is used to determine the symmetry of conformal coatings.
According to this embodiment, a sacrificial feature 21 is
generated, as shown in FIG. 7a and as shown in FIG. 7b a coating 22
is applied. The conformal coating is etched back to reveal the top
of the feature (FIG. 7c). The feature is then removed, usually by
etching to leave just the conformal layer feature, 23 (FIG. 7d) at
a lower pitch than the original feature. Radiation is then
projected onto the substrate and the reflected radiation detected.
The method then includes the following procedures:
[0066] a) obtaining fourier transform data (from the reflected
radiation data);
[0067] b) removing portions of the Fourier transform corresponding
to the conformal layer feature;
[0068] c) interpolating the remaining fourier transform over the
removed portions; and
[0069] d) subtracting the fourier transform of procedure (c) from
the fourier transform of procedure (a).
[0070] The resulting data can then be used to determine
characteristics of the coating and the substrate itself.
[0071] Alternatively, if the surrounding pattern is known, the
method may include procedures in accordance with the second
embodiment of the invention, namely:
[0072] (a) obtaining fourier transform data;
[0073] (b) performing a fourier transform on the known pattern;
and
[0074] (c) subtracting the fourier transform of procedure (b) from
the fourier transform of procedure (a).
[0075] An embodiment of the invention has been described primarily
in conjunction with an angle resolved scatterometer, although it
may also be used in conjunction with, for example, a spectroscopic
scatterometer or an ellipsometer.
[0076] Although specific reference may be made in this text to the
use of lithographic apparatus in the manufacture of ICs, it should
be understood that the lithographic apparatus described herein may
have other applications, such as the manufacture of integrated
optical systems, guidance and detection patterns for magnetic
domain memories, flat-panel displays, liquid-crystal displays
(LCDs), thin film magnetic heads, etc. The skilled artisan will
appreciate that, in the context of such alternative applications,
any use of the terms "wafer" or "die" herein may be considered as
synonymous with the more general terms "substrate" or "target
portion", respectively. The substrate referred to herein may be
processed, before or after exposure, in for example a track (a tool
that typically applies a layer of resist to a substrate and
develops the exposed resist), a metrology tool and/or an inspection
tool. Where applicable, the disclosure herein may be applied to
such and other substrate processing tools. Further, the substrate
may be processed more than once, for example in order to create a
multi-layer IC, so that the term substrate used herein may also
refer to a substrate that already contains multiple processed
layers.
[0077] Although specific reference may have been made above to the
use of embodiments of the invention in the context of optical
lithography, it will be appreciated that the invention may be used
in other applications, for example imprint lithography, and where
the context allows, is not limited to optical lithography. In
imprint lithography a topography in a patterning device defines the
pattern created on a substrate. The topography of the patterning
device may be pressed into a layer of resist supplied to the
substrate whereupon the resist is cured by applying electromagnetic
radiation, heat, pressure or a combination thereof. The patterning
device is moved out of the resist leaving a pattern in it after the
resist is cured.
[0078] The terms "radiation" and "beam" used herein encompass all
types of electromagnetic radiation, including ultraviolet (UV)
radiation (e.g. having a wavelength of or about 365, 355, 248, 193,
157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g.
having a wavelength in the range of 5-20 nm), as well as particle
beams, such as ion beams or electron beams.
[0079] The term "lens", where the context allows, may refer to any
one or combination of various types of optical components,
including refractive, reflective, magnetic, electromagnetic and
electrostatic optical components.
[0080] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. For example, the invention
may take the form of a computer program containing one or more
sequences of machine-readable instructions describing a method as
disclosed above, or a data storage medium (e.g. semiconductor
memory, magnetic or optical disk) having such a computer program
stored therein.
[0081] The descriptions above are intended to be illustrative, not
limiting. Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without
departing from the scope of the claims set out below.
* * * * *